The subject matter disclosed herein relates to computer tomography, and more particularly to determining quality of individual voxels in volumetric models.
Tomography refers to imaging by sections or sectioning, through the use of any kind of penetrating wave, such as x-rays. A device used in tomography is called a tomograph, while the image produced is a tomogram. There are two basic steps in tomography: 1) acquisition of x-ray projections of an object; and 2) reconstruction of the captured x-ray projections to create a volumetric representation of the object, in which the volume is constructed of “voxels”. A voxel (volumetric pixel or, more correctly, Volumetric Picture Element) is a volume element, representing a value on a regular grid in three-dimensional space. This is analogous to a pixel, which represents 2D image data in a bitmap (which is sometimes referred to as a pixmap).
During the acquisition phase, x-rays are emitted by an x-ray source from a focal spot, some of which penetrate the object. X-rays that penetrate the object are partially absorbed by the object. A detector records the x-rays and generates x-ray projections as digital images. The x-ray projections are usually saved on a computer hard drive or more generally in a computer memory. A detector can include a two dimensional array or a one dimensional array of pixels. Usually, to create a set of x-ray projections, the object is rotated at discrete intervals over 360° using the manipulation system, and at every rotational position one x-ray projection is generated. It is also possible to rotate the object over less than 360° or to rotate the object continuously.
During the reconstruction phase, the recorded x-ray projections are used to calculate the volume. Commonly used methods for reconstruction include but are not limited to: filtered backprojection, Feldkamp-Davis-Kress (FDK) iterative techniques (e.g., algebraic reconstruction technique (ART), simultaneous algebraic reconstruction technique (SART), and statistical methods (e.g., maximum-likelihood expectation-maximization (ML-EM)).
As described above, voxels are comparable to pixels but have x-y-z coordinates instead of x-y coordinates. In addition, each voxel describes the x-ray absorption coefficient at corresponding object position. Currently there is no suitable method for determining the quality of a voxel or how well a voxel is reconstructed (i.e., voxel confidence). There can be situations where artifacts can't be distinguished from real failures and details in the reconstructed volume. This leads to misinterpretation and detection problems with automated analysis algorithms.